The increasing commercial importance of rechargeable lithium ion battery cells has prompted a desire to identify and to prepare cathode materials better able to reversibly intercalate lithium ions at higher voltages. There are three prominent reversible lithium intercalation compounds used for lithium ion rechargeable batteries; namely, LiCoO.sub.2 and LiNiO.sub.2 compounds, and LiMn.sub.2 O.sub.4 spinel.
The present invention relates to a method of making hexagonal lithiated metal oxide materials at reduced temperatures. More particularly, the invention relates to a method of synthesizing lithium cobalt oxide or lithium nickel oxide products which is economical and which yields products having good electrochemical properties. The invention also relates to a method of producing a cobalt hydroxide precursor.
LiCoO.sub.2 cells are of particular interest because of their ability to insert/deinsert lithium reversibly at voltages greater than 4 V, resulting in batteries that have an output voltage and an energy density three times greater than Ni--Cd cells. Lithium cobalt oxide adopts a hexagonal structure consisting of CoO.sub.2 layers separated by Van der Waals gap. The octahedral sites within the Van der Waals gap are occupied by the Li ions. This results in the reversible intercalation of lithium. LiNiO.sub.2 is isostructural with LiCoO.sub.2 and is commercially viable for use in secondary lithium ion batteries.
Lithium secondary batteries are described for instance in U.S. Pat. Nos. 5,296,318 and 5,418,091 to Gozdz et al., both of which are incorporated in their entirety herein by reference. Lithium metal-free "rocking chair" batteries may be viewed as comprising two lithium-ion-absorbing electrode "sponges" separated by a lithium-ion conducting electrolyte usually comprising a Li.sup.+ salt dissolved in a non-aqueous solvent or mixture of such solvents. Numerous such salts and solvents are known in the art as evidenced in Canadian Patent Publication No. 2,002,191, dated Jan. 30, 1991.
U.S. Pat. No. 5,192,629, which is herein incorporated by reference in its entirety, provides a class of electrolyte compositions that are exceptionally useful for minimizing electrolyte decomposition in secondary batteries comprising strongly oxidizing positive electrode materials. These electrolytes are uniquely capable of enhancing the cycle life and improving the temperature performance of practical "rocking chair" cells. These electrolyte compositions have a range of effective stability extending up to about 5.0 V at 55.degree. C., as well as at room temperature (about 25.degree. C.).
A substantial cost in the fabrication of lithium secondary batteries is the cost of electrode material resulting from the price of Co- or Ni-based precursors plus the processing cost. Prior methods of synthesizing LiCoO.sub.2 include heating to temperatures of from 800.degree. C. to 900.degree. C. Reduction of the synthesis temperature of LiCoO.sub.2 would result in significant savings in the energy and cost in the production of these electrode materials.
Barboux et al., Journal of Solid State Chemistry, 94, (1191) 185, have reported a low temperature sol-gel approach to the synthesis of LiCoO.sub.2, but temperatures greater than 700.degree. C. are still necessary to obtain poorly crystalline powders of LiCoO.sub.2. R. J. Gummow et al., Mat. Res. Bull., 27 (1992), 327, and E. Rossen et al., Solid State Ionics, 62 (1993) 53, tried to prepare LiCoO.sub.2 at low temperature (400.degree. C.) from CoCO.sub.3 and obtained a compound which they called "LT LiCoO.sub.2 ". This material adopts a spinel (cubic) rather than an hexagonal structure. The LT LiCoO.sub.2 phase, which does not present any interest from an electrochemical point of view, transforms to the hexagonal LiCoO.sub.2 phase at temperatures greater than 600.degree. C. Such a LiCoO.sub.2 spinel structure results most likely from the fact, as suggested by Barboux et al., that the phase grows or nucleates from the cubic Co.sub.3 O.sub.4 spinel.
Reimers et al., in J. Electrochem. Soc. (May 1993), reported a low-temperature synthesis method for LiMnO.sub.2. The material produced by Reimers et al. at low temperatures, e.g., 400.degree. C., however, was unlike lithium manganese oxide produced at high temperatures and exhibited inferior electrochemical properties. Other low temperature processes have been attempted; for example, Fernandez-Rodriquez et al., in Mat. Res. Bull., Vol. 23, pp. 899-904, report unsuccessful attempts to form LiCoO.sub.2 from HCoO.sub.2 at 200.degree. C.